1. Field of Invention
The invention relates to a communication system and the method thereof. In particular, the invention relates to a communication system across networks and the corresponding method that can implement remote controls of devices in a private network from the client end of a public network.
2. Related Art
Due to prosperous Internet developments in recent years, networking devices requiring IP addresses also grow rapidly. The IP addresses are unique. Moreover, the widely used IP addresses are in accord with the fourth-generation protocol (IPv4), which only uses 32 bits to locate all networking devices worldwide. Therefore, the IP addresses are insufficient for current uses.
In observation of this, some vendors provide the network address translation (NAT) technique to make full use of the limited amount of IP addresses. Generally speaking, the IP addresses include public IP addresses and private IP addresses. The so-called public IP address is a legal IP address issued by an Internet service provider (ISP), i.e., a physical network address on the Internet (the public network). The private IP address is an IP address that is reserved and not issued, also called a virtual IP address. The reserved IP addresses are for an intranet (the private network). They can be arranged and issued by the network administrator according to needs. For example, the reserved IP address according to IPv4 range from 10.0.0.1 to 10.255.255.254, from 172.16.0.1 to 172.31.255.254, and from 192.168.0.1 to 192.168.255.254. A same private IP address can be used in different private networks. Therefore, this method can effectively solve the problem of insufficient IP addresses. However, since different private networks may have the same private network addresses, packets cannot be freely transferred between private and public networks in order to avoid incorrect addressing. To transmit, packets have to rely on an intermediate end between the private and public networks and supporting IP address translations, such as an IP router.
However, the above-mentioned IP address translation requires a first device in the private network to sends a connection request to a second device in the public network, so that the correspondence relation between IP address of the first device and the second device is recorded in a correspondence table at the intermediate end during the connection request. The correspondence table is then used for packet transfers. Therefore, if the first device does not send a connection request to the second device, it is impossible for the second device in the public network to perform a remote control on the first device in the private network.
In summary, the prior long has the problem that the second device in the public network cannot actively perform remote controls on the first device in the private network. It is therefore highly desirable to provide a solution.